This invention relates to transmitters and particularly to small, low voltage (1.5 to 6 volts nominal) transmitters for transmitting at a single selected frequency, normally (but not essentially) at a very high frequency (VHF), that is greater than 30 MHz.
In the design of transmitters which are used for the location or tracking of birds, animals, vehicles, people or other objects it is desirable to use as small and light a transmitter as is possible.
The invention is particularly concerned with transmitters for use in the sport of falconry. For such sport it is common for small transmitters to be attached to hunting birds in order that they can be tracked and located if lost or out of sight. For this use it is particularly important that the transmitters are small and light while remaining capable of emitting as strong a signal as possible. It is also desirable that their active life is of several days (at least four).
The frequencies used for this sport are designated in each country in which it is praticed. In the UK the legal frequency granted by the government is approximately 173-174 MHz. In USA, Canada, Europe and the Middle East the frequency alloted for falconry is approximately 216 MHz. In Germany frequencies of 151 to 155 MHz and 403 to 405 MHz are also used.
The transmitter circuit is usually based on a crystal oscillator having a predetermined nominal frequency, a frequency multiplying and amplifying circuit and a pulsing circuit designed to give a pulsed output at the predetermined frequency. The transmitters normally have a power less than 50 milliwatts and are driven by batteries at 3 or 4.5 volts nominal.
The transmitter circuit most commonly used at present in falconry is that shown in FIG. 6. In this circuit the crystal oscillator stage O oscillates at a frequency approximately one third that required (216 MHz) and a frequency multiplying circuit stage M is arranged to triple the frequency. This known circuit has a pulsing stage P providing an OFF period in which no signal is transmitted and an ON period in which a signal is transmitted. The OFF period is defined basically by a capacitor charging up through a resistor and the ON period by the same capacitor discharging through another resistor and a transistor which forms part of the oscillator stage. For this known circuit to function correctly a good DC gain characteristic is required for the transistor. For efficient oscillation, however, that transistor also needs to have a good gain characteristic at the crystal frequency. These requirements are often conflicting and the result is a wide amptitude variation of output signal from the oscillator stage and thus from the transmitter. The frequency tripling stage, mentioned above, is chosen so that the oscillator stage can function at a lower frequency where it is possible to obtain transistors with a sufficient DC gain to operate the pulsing circuit, but this is less efficient than, for example, a frequency doubling circuit. There is no means for tuning the frequency once assembled.
An object of the present invention is to provide a more efficient transmitter circuit which will provide a higher output power for less battery power, which is small and light and which is preferably reproducable from one example to another using standard components.